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Orange County-Poughkeepsie Limited Partnership (Narrative) (Details) (USD $)
3 Months Ended 12 Months Ended 0 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Dec. 31, 2012
Apr. 30, 2014
Option Indexed to Issuer's Equity [Line Items]        
Equity interest in O-P 8.108%us-gaap_EquityMethodInvestmentOwnershipPercentage      
Equity method investment, amount the investment account was reduced to $ 0altv_EquityMethodInvestmentAmountInvestmentAccountWasReducedTo $ 0altv_EquityMethodInvestmentAmountInvestmentAccountWasReducedTo $ 0altv_EquityMethodInvestmentAmountInvestmentAccountWasReducedTo  
Put Option [Member]        
Option Indexed to Issuer's Equity [Line Items]        
Proceeds from exercise of stock options       50,000,000us-gaap_ProceedsFromStockOptionsExercised
/ us-gaap_OptionIndexedToIssuersEquityEquityAxis
= us-gaap_PutOptionMember
Proceeds on sale of ownership interest       $ 49,800,000us-gaap_ProceedsFromSaleOfEquityMethodInvestments
/ us-gaap_OptionIndexedToIssuersEquityEquityAxis
= us-gaap_PutOptionMember